There's a product for this. It's designed to go over asphalt shingle and they call it... wait for it... "roof coating". Here's a link to the manufacturers' trade group: http://www.roofcoatings.org/wcc.html [roofcoatings.org].

How long before we here the politicians whining that Clinton's trying to outlaw roof shingles or make everyone replace their roof.

I'd use white roof coating, rolled on some test shingles nailed to a sheet of plywood before trying any specific coating on a home. It may take a couple of coats to cover. I would also consider a light gray coating if available.

The temp reduction is no joke. I coated the tops of my ISO container shop buildings and my metal house roof white. The ISOs are sealed, no windows (stormproof and keeps my gear dry) and get rather warm. White tops make them reasonable workspaces.

I didn't add the glass microspheres one can buy to mix with paints and roof coatings, but I'll do that next time. (BTW I fucking HATE asphalt shingles. If I won the lottery I'd use steel trusses and modern standing seam roofing.)

The way to do it is not to assume it'll be in the news for one day, maybe for a month people will do it, and then back to American Idle.

The way to do it is to create a federal, state, and local bureaucracy of roof painting enforcement, to require city building permits (with inspections) for all exterior painting. This I suppose requires govt oversight at all levels to license painters, and of course trainers of painters, and trainers of trainers of painters. Create a couple federal jobs programs to funnel money from the govt to campaign contributors while almost accidentally having a couple disadvantaged youth paint a roof or two as a side effect. Remove federal highway funding from any state that does not force its cities building codes to be modified. Then too we need a national census with hired workers to verify roof painting status. And a federal dept to collect all utility records from all americans to analyze for the white roof savings, and to do paramilitary operations on grow ops. People that want "normal" shingles or historical shingles will have to sneak them across the Canadian border, just like people that want real toilets do now. That is a realistic conspiracy theory.

For most materials absorption and emissivity of IR is usually the same for any given frequency.

Paint: Krylon, flat white #1502 @ 3m wavelenght = 0.992 emissivitySo weirdly enough this one specific "Visibly" white paint has one of the highest emissivities, and would absorb and reflect a lot of heat almost the same as the black paints!!!

The nice disadvantage of covering a roof with solar panels is that in case of a fire barely or no water will reach the fire.

If your roof lets water in, yr doing it wrong. The whole point of a roof is to not water through.

(The reason that the firefighters spray water on a roof in the first place is to keep old-fashioned asphalt shingles from igniting-- if the roof is fireproof, that's a feature, not a bug. By the time the roof has burned through enough to let water into the house, it's pretty much too late to save the building.)

I suspect they would actually make the building warmer. It's a common misconception that black absorbs heat while white reflects it. Well, that in itself isn't the misconception, the misconception is that people think of the heat transfer happening only one way. Black is more conducive to heat transfer [wikipedia.org] than white. That means if you have a black roof in winter (assume no snow), and the inside of the house is warmer than the outside, the black roof will radiate the interior heat to the sky faster than would a white roof. Think of white paint as a heat insulator, and black paint as a heat conductor.

True, during daylight the radiation from sunlight into the roof may exceed the radiation from inside the house to outside. But days are short in Winter, and I suspect the heat loss during the night far exceeds the gain during the day.

I think you're getting confused. What matters most is reflectivity, not emissivity. The solar spectrum is most intense in the visible part of the spectrum (no surprise - that's why our eyes evolved to be sensitive to that part of the spectrum), and white paint is much more reflective in that part of the spectrum than black paint (by definition - that's what it means to be white!).

And why are you quoting the emissivity of a particular paint at 3m wavelength? That's radio frequency. Solar energy in that frequency range is completely negligible compared to visible.